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9551 Datasheet PDF : 26 Pages
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NXP Semiconductors
PCA9551
8-bit I2C-bus LED driver with programmable blink rates
Table 5.
Bit
Symbol
Default
PSC0 - Frequency Prescaler 0 register description
7
6
5
4
3
2
PSC0[7] PSC0[6] PSC0[5] PSC0[4] PSC0[3] PSC0[2]
1
1
1
1
1
1
1
PSC0[1]
1
0
PSC0[0]
1
6.3.3 PWM0 - Pulse Width Modulation 0
The PWM0 register determines the duty cycle of BLINK0. The outputs are LOW (LED off)
when the count is less than the value in PWM0 and HIGH when it is greater. If PWM0 is
programmed with 00h, then the PWM0 output is always LOW.
The duty cycle of BLINK0 = (256 − PWM0) / 256.
Table 6.
Bit
Symbol
Default
PWM0 - Pulse Width Modulation 0 register description
7
6
5
4
3
2
PWM0 PWM0 PWM0 PWM0 PWM0 PWM0
[7]
[6]
[5]
[4]
[3]
[2]
1
0
0
0
0
0
1
PWM0
[1]
0
0
PWM0
[0]
0
6.3.4 PSC1 - Frequency Prescaler 1
PSC1 is used to program the period of the PWM output.
The period of BLINK1 = (PSC1 + 1) / 38.
Remark: Prescaler calculation is different between the PCA9551 and other PCA955x
LED blinkers. A divider ratio of 38 instead of 44 is used. This different divider ratio causes
the blinking frequency to be 13 % (1 − 38 / 44) lower when the same 8-bit word is used.
The programmed value of Frequency Prescaler 1 must be adjusted to compensate for this
difference in applications where the PCA9551 is used in conjunction with other PCA955x
LED blinkers and the observed blinking frequencies need to be the same.
Table 7.
Bit
Symbol
Default
PSC1 - Frequency Prescaler 1 register description
7
6
5
4
3
2
PSC1[7] PSC1[6] PSC1[5] PSC1[4] PSC1[3] PSC1[2]
1
1
1
1
1
1
1
PSC1[1]
1
0
PSC1[0]
1
6.3.5 PWM1 - Pulse Width Modulation 1
The PWM1 register determines the duty cycle of BLINK1. The outputs are LOW (LED off)
when the count is less than the value in PWM1 and HIGH when it is greater. If PWM1 is
programmed with 00h, then the PWM1 output is always LOW (LED off).
The duty cycle of BLINK1 = (256 − PWM1) / 256.
Table 8.
Bit
Symbol
Default
PWM1 - Pulse Width Modulation 1 register description
7
6
5
4
3
2
PWM1 PWM1 PWM1 PWM1 PWM1 PWM1
[7]
[6]
[5]
[4]
[3]
[2]
1
0
0
0
0
0
1
PWM1
[1]
0
0
PWM1
[0]
0
PCA9551_8
Product data sheet
Rev. 08 — 31 July 2008
© NXP B.V. 2008. All rights reserved.
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